In data processing systems, magnetic disc memories are frequently used because they have a high storage capacity and require a relatively short time to access magnetic read/write heads to a data item contained anywhere on a disc after the head receives an instruction to access the data item from the data processing system.
Magnetic discs carry data on both faces of the disc. On each face, the data are arranged in concentric, circular record tracks, each having a width that does not exceed a few hundredths of a millimeter. The tracks are identified by assigning each of them a serial number j between 0 and (N-1) (where j is a whole number and N is the number of tracks on each face). An address is a coded expression of a track serial number j. Typically, addresses and other data are recorded in binary codes. Only one reading head, i.e. reading transducer, is generally associated with each face of a disc, but each disc face is associated with one head, whereby two heads are associated with each disc.
A series of data items on a magnetic disc is recorded in each track as a succession of small magnetic domains (referred to as "elementary magnets") distributed along the entire track length. Each magnetic domain has a magnetic induction in a direction parallel to the disc surface, and successive domains have opposite magnetic senses or polarities.
The term "bit" designates a binary one or binary zero digit. A binary digit can be expressed as an analog or logic electric signal. An analog signal is defined as a signal having a voltage that varies continuously between two limits, one of which is generally positive and the other negative. A logic signal is capable of assuming only two values, referred to as a logic zero or a logic one value.
Memories having a relatively small storage capacity, containing only one or two discs, have track addresses recorded on one face of a disc so that a maximum amount of space is set aside for data to be processed by the data processing system. A relatively small amount of space on each face is set aside for reference zone addresses and magnetic flux variations that assist in servo-controlling the magnetic head associated with the face to a position on the face. The addresses and servo position control fluxes, frequently referred to as items of track locating data, are generally recorded in reference zones distributed over the entire disc surface. The number of reference zones is at least equal to the number of tracks, so that each track has at least one zone associated with it. Each zone includes an indexing indicia or first marker, enabling the beginning of the zone to be located, and which is followed by a series of individual cells, each of which contains one item of track locating data, i.e. servo position flux variations and address bits. The "beginning of a reference zone" is defined as a set of data items, usually the servo position data, in the zone which are initially read by the head when the face containing the reference zone moves past the head. Similarly, the "remainder of the zone" is defined as the set of data items, usually the zone address, in the zone which are read last by the head, i.e. after the set of items which are read first. Generally, the number of data items contained in the beginning of the zone is less than the number of data items contained in the remainder of the zone.
In each address bit cell of a magnetic disc, a binary bit is represented by a change in magnetization sense. The magnetization sense change occupies one of only two predetermined positions within the cell so the value of the item associated with the cell depends upon the position where the change occurs within the cell. Changes in magnetization sense must be repeated identically from one cell to the next, so that the magnetization sense of a first elementary magnet in each cell is always the same. Thereby, each address bit cell must include a second change in magnetization sense, referred to as an adjusting change.
In response to a magnetic reading head of a magnetic disc encountering a series of magnetization sense changes corresponding to a reference zone, the head derives a series of analog waveforms that are shaped into logic pulses by shaping circuits. The beginning of each reference zone is indicated by a special pulse, termed a first marker pulse.
Track locating data waveforms derived by the magnetic head are supplied to an apparatus that positions the head above the disc. The apparatus enables the head to be moved radially from track A, where the head is initially positioned, to track B, from which it is desired to read data. The head positioning apparatus also enables the magnetic head to be held exactly above the middle of track B during the time required to read data from track B. To enable data from track B to be read by the head as quickly as possible and with maximum accuracy, it is important for the time required to move the head from track A to track B be as short as possible and that the head be positioned above the center of track B as accurately as possible.
As the head moves over the track past the servo position data, the head derives a waveform having an average value, with undulations about the average value. As each cell is traversed, a properly positioned head derives a waveform having a pair of positive sinusoidal like equal amplitude waveforms that extend above the average value during the first and second third of each cell. The sinusoidal like waveforms are followed during the last third of the cell by a single negative pulse having an amplitude considerably greater than the amplitude of the sinusoidal waveforms. If the peak amplitudes of the sinusoidal waveforms during the first and second thirds are not substantially equal, an indication is derived that the head is not properly positioned over the center of the track. A servo system responds to the inequality to position the head properly. Typically, six cells of this type are provided at the beginning of each reference zone to properly position the head on the track.
The servo position cells are followed by the cells for the address bits of the reference zone. The properly positioned head reads each reference zone address cell bit as a waveform having the same average value as the waveform during the servo position cells. The waveform read from the first address cell always includes a pair of equal amplitude, sinusoidal like waveforms that extend above the average value. Thereafter, the cells represent the reference zone address bits. The waveform derived for each address bit cell includes two equal duration segments. Normally, one segment includes a relatively flat portion at the average value and the other segment includes a pulse, having a relatively large amplitude, approximately equal to the pulse amplitude in each of the servo position cells. The position of the pulse in each cell indicates whether the cell bit value is a binary zero or one. In a particular embodiment, pulses in the first and second halves of each cell respectively represent binary zero and one values. Because each cell must include a pair of opposite polarity magnetic domains, the pulse polarity for adjacent cells must be in opposite directions. For example, the pulses associated with even and odd numbered address cells are respectively in the negative and positive directions in the first or second half of each cell, so that binary ones in even and odd numbered cells are respectively represented as negative and positive pulses in the second half of the cells while binary zeros in the odd and even cells are respectively represented as positive and negative pulses in the first half of the cells.
To enable the head to be moved from track A to track B in the shortest possible time and to position the head properly over the center of track B, any analog waveform corresponding to any magnetization sense change in a reference zone must be detected with maximum accuracy. Hence, the head analog waveform only represents an item of servo position information or an address bit if the negative pulse during the last third of each initial cell or an undulation during the first or second half of an address cell reaches a certain threshold level. In prior art systems, the threshold is set at a relatively low, arbitrary value that represents only approximately 20-25% of the maximum amplitude which an analog waveform derived by the head is able to attain. This is because the amplitude of waveforms read from the head varies as a function of relative head and disc velocity, in turn a function of a number of variables, e.g. head position on the disc. Generally, the waveform amplitude increases as the head is moved toward the disc periphery, i.e., away from the disc center, because the disc-head linear velocity increases as head radius increases. But the waveform amplitude can vary as a function of circumferential position, for a constant radius. Hence, setting an arbitrary maximum threshold is likely to result in errors because certain waveforms may not have sufficient amplitude to exceed the present arbitrary amplitude, unless the amplitude is set at the very low level of 20-25% of maximum. On the other hand, relatively low noise in the track having high peak amplitudes is likely to have sufficient amplitude to exceed the arbitrary threshold level.
It is, therefore, an object of the present invention to provide a new and improved system for and method of setting the threshold of waveforms read by a magnetic transducing head, which waveforms are subject to substantial amplitude variations, even though they are designed to represent binary values.
A further object of the invention is to provide a new and improved system for and method of accurately reading binary data from reference zones at different radial positions of a magnetic disc.